Wustite texture and microstructure modification by ceria surface addition during low pressure oxidation of pure iron at high temperatures

Modification de la texture et de la microstructure de la wüstite par un dépôt superficiel d’oxyde de cérium

Abstract

Pure iron oxidation is studied in the oxygen partial pressure ranging between p(O₂) = 2.10^-4 Pa and p(O₂) = 4.10^-2 Pa. These pressures were chosen in order to insure wustite formation in the temperature domain 700-950°C. Textural analysis with pole figure plotting is performed on specimens oxidized at 800 and 950°C in oxygen (partial pressure of p(O₂) = 2.10^-3 Pa). We have demonstrated that cerium coatings modify the microstructure and texture of a wustite scale grown on pure iron. All the resuits indicate that the main effect of the coating is to prevent the epitaxial relations between iron and the wustite scale. Then, the whole surface is covered by a fast growing scale. The pole figures show that the (200) planes of the wustite structure are mainly oriented parallel to the specimen surface. The [200] orientation is always associated to the faster growing scale. On blank specimens, the slow kinetic rate is attributed to the fact that the main part of the iron surface is covered by a wustite scale with orientation distinct from [200]. A good scale adherence was observed since the coating was sufficiently thin and regular to permit the (CeFeO₃) cerium orthoferrite formation from the ceria coating. A perowskite type oxide appears as a good candidate for an homogeneous cerium introduction into oxide scales. Its formation is favoured by the iron external diffusion which looks like easy in the proceeding conditions.